Believe it or not, there's a crowdsourced, open source non-profit attempting to build a sea-launched suborbital rocket. Called Copenhagen Suborbitals, it even had access to a submarine. A club associated with the venture completed the sub in 2008, designed by Peter Madsen, a Danish inventor who is co-founder of the group. That submarine is now at the bottom of the sea, and Madsen is being held by Danish authorities on suspicion of "unlawful killing"—a precursor charge to manslaughter or murder.

The UC3 Nautilus was the third and largest submarine effort by the club, costing $200,000 to construct. It served as a workhorse for Copenhagen Suborbitals, helping push the group's Sputnik rocket launch platform into position on a number of occasions. Nautilus is—or was—powered by two diesel engines above the surface and by batteries underwater. While it could hold a crew of four underwater, all of its controls could be managed by a single person from its control room.

By 2011, the sub needed an overhaul. But the repairs required more than Copenhagen Suborbitals could afford to sink into the Nautilus. So in 2013, the group launched an Indiegogo campaign to get it back in the water. In a video, Madsen described the sub and the inspiration behind it.

Madsen's pitch for crowdfunding repairs on the UV3 Nautilus, from 2013.

The crowdfunding project fell short, and Madsen eventually took possession of the Nautilus himself after an often contentious dispute over control and ownership of the sub, continuing work on it himself. (Madsen is no longer associated with Copenhagen Orbital). His efforts drew the attention of a Swedish journalist, who took a trip with Madsen on the sub this week. The reporter had said she was writing a story for Wired (though she was not on assignment for Wired in the US, and was not formally employed by the UK edition of the publication). Madsen was seen departing with her around 7pm local time on August 10 from Refshale Island (Refshaleøen), an industrial area east of Copenhagen. (Update: the journalist has been identified as Kim Wall, who has contributed in the past to the Guardian and the New York Times.)

At 3:30am Friday, the Copenhagen Police received a call from Denmark's Joint Rescue Coordination Center (JRCC) indicating that the Nautilus had not returned from what was supposed to be a trial run. About 7 hours later, a harbormaster reported spotting the submarine passing Drogden Lighthouse, in Køge Bay, well south of Copenhagen's harbor. Madsen spoke with the harbormaster, via radio, indicating that he was headed for the harbor. When asked why he had not been in contact with the harbor, he said that he had been experiencing technical issues.

But he never made it back to the harbor. According to the police report, "At 11.00 the submarine suddenly sank and the owner was subsequently rescued on a private motorboat sailing [near] him in port."

After being rescued, Madsen told Denmark's TV2 that "a minor problem with a ballast tank... turned into a major issue." The ballast tank, which holds air or water to vary the submarine's buoyancy, apparently filled with water unexpectedly. The Nautilus' hatches were open, so as the sub started to submerge it began to flood. Madsen said that the sub was gone in 30 seconds. If he had been below decks instead of in the small mast of the sub, he would have been killed.

But Madsen didn’t mention his passenger. When questioned by the police, he said that he had dropped the woman off Thursday evening near where they had departed from, around 10:30pm. But no one had been able to reach the woman or establish her whereabouts.

Police are still seeking anyone who may have seen the woman or the submarine on Thursday night. A search effort was still underway as of this evening, and the submarine has been located at the bottom of the bay in 7 meters (22 feet) of water, but divers have not been able to enter the sub to check for a body.

Update [August 14, 10:00 am Eastern]: The Nautilus has been raised, but no body was found aboard. However, police are continuing to hold Madsen, as he has changed his story about dropping Wall off. Details of his new narrative have not been publicly shared. Madsen will be held an additional 24 days while an investigation continues. Police believe Madsen deliberately sank the sub.